Lithium-ion batteries (LIBs) have been considered as one of the most representative classes of modern secondary batteries since their debut in 1990 due to superior features such as high energy density, high discharge potential, low self-discharge, and no memory effects in comparison to traditional rechargeable cells. The prevalence of lithium-ion batteries in numerous applications from wearable electronics to electric vehicles raises increasing attention to safety issues. One of the major concerns with lithium ion batteries is overcharge, which can result in highly dangerous potential hazards like battery component damage, overheat, burn, and even explosion. Overcharge of lithium-ion batteries is a continuous electricity input to cells when full capacity has been reached. Instead of being stored in the electrode, redundant electric energy resulting from overcharge tends to accumulate on the surface of the electrode and elevate the potential dramatically, leading to exothermic reactions of electrolytes and other battery components that are electrochemically inert in normal the potential range of the charging process. Currently, most practical overcharge protection methods can be categorized into two major types: (1) electronic overvoltage cut-off devices, and (2) redox shuttle additives for chemical overcharge protection. It is noteworthy that the second type has elicited particular attention from researchers due to a number of comparative advantages including lower cost, minimized additional weight and volume, and inherent overcharge suppression.
Generally, the redox shuttle molecule can be reversibly oxidized and reduced at a defined potential slightly higher than the end-of-charge potential of the cathode. This mechanism can protect the cell from overcharge by locking the potential of the cathode at the oxidation potential of the shuttle molecules. On the overcharged cathode surface, the redox shuttle molecule (S) is oxidized to its (radical) cation form (S⋅+), which, via diffusion through the electrolyte, is reduced back to its original or reduced state on the surface of the anode. The reduced form can then diffuse back to the cathode and oxidize again. The “oxidation-diffusion-reduction-diffusion” cycle can be repeated continuously due to the reversible nature of the redox shuttle to shunt the overcharge current. The redox shuttling mechanism at overcharge can be regarded as a controlled internal short, and the net result of the shuttling is to convert the overcharge electricity power into heat, which avoids the reactions that occur between the electrodes and electrolyte at high voltage. Redox shuttles can also be used for automatic capacity balancing during battery manufacturing and repair. Modern LIB designs are challenging the limits of current redox shuttle materials.
Ideal redox shuttle additives are materials that can be readily dissolved in electrolyte and have appropriate oxidation potential (approximately 0.3-0.5 V higher than that of the cathode) while maintaining high electrochemical stability. Once the potential of the cathode exceeds the potential of redox shuttle additives, initially unreactive shuttle molecules will be electrochemically activated, and start to take over the oxidative electrons. As a result, the potential of the cell remains unchanged in this process. Redox shuttle additives in the oxidative state then diffuse to the anode and get reduced to the initial state, which will transport back to the cathode. Therefore, this sustainable circle of redox shuttle additives prevents potential hazards of overcharge by fixing the cathode potential at their oxidation potential. Although a large variety of materials have been attempted to serve this purpose, it still remains a great challenge to develop shuttle additives featuring high solubility, suitable oxidation potential, and high electrochemical stability in one system. To this day, one of the most promising additives is 2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB), which has an oxidation potential at 3.98 V vs. Li/Li− and undergoes overcharge abuse for over 3000 hours before a significant drop in performance is observed. One possible reason for the eventual performance drop is believed to be the consumption of redox shuttle additives through reactions between the two unsubstituted positions on the radical cation after oxidation.
There is an ongoing need for new redox shuttle chemistries to ameliorate the overcharge phenomenon in lithium-ion batteries. The redox shuttle additives described herein address this need.